Breathing frequency has been shown to depend on metabolic demand, and also to be affected by stepping frequency during walking. To assess the influence of stepping frequency on breathing frequency, we recorded the timing of breathing and stepping in ten naive subjects walking on a treadmill. Five of the subjects showed periods of breathing in rhythm with their stepping (rhythmical entrainment); five subjects never showed rhythmical entrainment. In all subjects, respiratory frequency tended to increase with increases in estimated metabolic rate (EMR) produced by increases in walking speed or treadmill inclination. Breathing frequency was also affected by stepping frequency independent of EMR, both during rhythmical entrainment and in the absence of rhythmical entrainment. The dependence of breathing frequency on stepping frequency may reflect an excitatory influence of voluntary movement on the respiratory center. We conclude that stepping importantly influences breathing frequency during walking whether or not the two are rhythmically linked. Such lability of respiratory frequency probably reflects the small metabolic cost of departure from mechanically optimal frequencies.